1. Field of the Invention
The present invention relates to a metrology instrument and metrology method for inspecting whether elements of a pattern are stitched correctly at their joint within an image and, if there is a deviation between them, for measuring the amount of the deviation.
2. Description of the Related Art
Electron beam lithography methods include the block exposure method (also known as the cell projection method) using a mask having various kinds of apertures for improving the throughput. In particular, one of the apertures is selected, and an electron beam is partially illuminated onto a target through the aperture to transfer a desired pattern. In this method, pattern sections formed by the partial illumination are successively stitched together, thus forming the whole pattern. Therefore, the stitching accuracy for the pattern must be high. It is necessary to measure the amount of deviation at each stitching boundary.
Examples of shapes of stitched portions of patterns are shown in FIGS. 2(a)-2(e). FIG. 2(a) shows the upper half 26 of a pattern to be stitched. FIG. 2(b) shows the lower half 27 of the pattern. FIG. 2(c) shows the manner in which the upper half 26 and the lower half 27 of the pattern have been normally stitched to form a straight pattern (transferred pattern) 28. FIG. 2(d) shows the manner in which the upper and lower straight portions of a pattern are not correctly stitched at a joint or a boundary 24 but form a distorted transferred pattern 28. FIG. 2(e) shows the manner in which the upper portion 26 and the lower portion 27 of a pattern are stitched at a joint with deviations ΔX and ΔY in the X- and Y-directions, respectively, to indicate their relations to the transferred pattern 28. This transferred pattern 28 shows a pattern actually formed on a wafer. On the other hand, the upper portion 26 and the lower portion 27 of the pattern can be considered to show elements of a prototypic pattern to be formed. Accordingly, in FIG. 2(e), the actually formed transferred pattern 28 is indicated by solid lines, while the upper portion 26 and the lower portion 27 of the pattern which indicate the elements of the prototypic pattern are indicated by broken lines. A straight boundary line 24 running along the joint or the stitching part is also shown.
The edges of the formed pattern become blunted because of scattering of electrons within the photosensitive material. This phenomenon is known as the proximity effect, which is illustrated diagrammatically in FIG. 3. Primary electrons 2 emitted from an electron gun (not shown) are made to hit a photosensitive material 18 on a substrate 17. The primary electrons 1 penetrate into the photosensitive material 18 according to the energies that the electrons possess, go through the layer of this photosensitive material 18, and even reach the substrate 17. In electron-beam lithography equipment, an accelerating voltage of 50 keV is normally used. During this process, the photosensitive material 18 is subjected to multipath exposure by elastic scatterings of electrons including forward scattering 19 caused in the photosensitive material 18 and backward scattering 20 caused on the substrate 17. As a result, the edges of the formed pattern become blunted (less sharp).
In measuring the stitching accuracy at the joint of such elements of a pattern, the following problem takes place. As shown in FIG. 2(e), the amount of horizontal deviation ΔX (i.e., deviation normal to the pattern shape) can be accurately calculated if the edges of the pattern are detected at the upper portion 26 and the lower portion 27 that are stitched. On the other hand, the amount of vertical deviation ΔY (i.e., deviation parallel to the pattern shape) cannot be easily measured because the edges of the transferred pattern 28 are blunted by the aforementioned proximity effect.
In measuring the stitching accuracy at such a pattern joint by one conventional technique, verniers 21 and 22 extending in the X- and Y-directions, respectively, are transferred inside and outside a rectangular pattern as shown in FIG. 4. As shown in FIG. 5, the stitching deviation (or error) ΔX at the joint 24 of the patterns 23 a and 23b is calculated from the results of detection of the edges of the two patterns 23a and 23b using line profiles 25a and 25b that are derived from scanning waveforms by detecting secondary electrons.
Another conventional method uses two transferred box-shaped patterns, i.e., an outer box-shaped pattern 29 and an inner box-shaped pattern 30, of different sizes as shown in FIG. 7(a). The relative positional relations (x1, x2, y1, y2) between the transferred outer box-shaped pattern 29 and inner box-shaped pattern 30 are measured, as shown in FIG. 7(b). Then, the stitching accuracy is calculated. This is known as the box-in-box method.
With any of the two conventional methods described above, however, it is necessary to previously make a sample pattern used for measurement of the deviation at the joint. An exorbitantly long time is taken to readjust the electron-beam lithography equipment for eliminating the stitching error. Consequently, it is impossible to measure the stitching error by the use of an actual logic pattern.